1. Field of the Invention
This invention relates to a novel aromatic polycarbonate and to an electrophotographic photosensitive medium having a photosensitive layer containing the aromatic polycarbonate.
2. Description of the Prior Art
As is known in the art, aromatic polycarbonates have been prepared from 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as "bisphenol A"), phosgene or biphenyl carbonate. Such aromatic polycarbonates have been utilized in various fields due to their superior transparency, heat resistance, dimensional stability and mechanical strength. For example, various studies have been made to apply these aromatic polycarbonates to binder resins for an organic photoconductor (OPC) which has been recently utilized in electrophotographic copying apparatuses, printers, etc. Typically, the organic photoconductor has a laminated structure composed of a conductive substrate, a charge-generating layer (CGL) provided over the conductive substrate and a charge-transport layer (CTL) provided over the charge-generating layer. The charge-transport layer is composed of a low-molecular charge-transport material (CTM) and a binder resin. Many aromatic polycarbonate resins have been proposed so as to be applied to the binder resin for the charge-transport layer. However, when the low-molecular charge-transport material is incorporated into the binder resin for the charge-transport layer, there occurs deterioration of an inherent mechanical strength of the binder resin. Consequently, the photosensitive member is likely to exhibit low wear resistance and poor durability, and suffer from scratches or cracks.
There have been hitherto proposed photoconductive polymers including vinyl polymers such as polyvinyl anthracene, polyvinyl pyrene, poly-N-vinyl carbazole or the like. These polymers all are of a so-called charge-transport complex type. These photoconductive polymers are however unsatisfactory in sensitivity to light.
On the other hand, attempts have been made to improve properties of the aforementioned laminate-type photosensitive member. As a result, many polymer materials having a high charge-transport capacity have been proposed. Examples of the polymer materials include acryl resins having a triphenylamine structure (M. Stolka et al. "J. Polym. Sci.,"Vol. 21, p.969 (1983)), vinyl polymers having a hydrazone structure ("Japan Hard Copy," (1989) p.67), polycarbonate resins having a triarylamine structure (U.S. Pat. Nos. 4,801,517, 4,806,443, 4,806,444, 4,937,165, 4,959,288, 5,030,532, 5,034,296 and 5,080,989, Japanese patent application laid-open publications Nos. 64-9964(1989), 3-221522(1991), 2-304456(1990), 4-11627(1992), 4-175337(1992), 4-18371(1992), 4-31404(1992) and 4-133065(1992), or the like). However, these polymer materials have not yet been put into practical use.
M. A. Abkowitz et al compare high molecular weight polycarbonate with low molecular weight one using a tetraarylbenzidine derivative as a model compound and conclude that high molecular weight polycarbonate shows the smallest drift mobility (Physical Review, B46, 6705(1992)). This conclusion suggests that high molecular weight polycarbonate has a problem in electrical characteristics such as sensitivity and residual potential.